1. Field of the Invention
The present invention relates generally to the field of phonograph sound reproduction and, more particularly, to the electromagnetic control of low-friction straight-line tone arms in a phonograph.
2. Description of the Prior Art
The advantages of utilizing radial or linear tone arms rather than pivotal tone arms in the audio reproduction of phonograph records is well documented. The master records from which the copies are pressed are produced by a cutter which describes a spiral groove by moving in a radial path with respect to the center of the record as the master is rotated at constant speed. Thus, the most accurate subsequent reproduction by a phonograph occurs where the stylus also moves radially with respect to the center of the record, i.e., with the axis of the tone arm perpetually tangent to the grooves. In addition, it is desirable that the tone arm, and thus the stylus, be as free to move radially as possible so as it precisely follows the groove. If the tone arm tends to "skate" inward toward the center of the record or tends to "drag" toward the outside, serious audio distortions and/or excessive record wear may result from the failure of the stylus to properly center in the groove.
It is also an established fact that both the records themselves and the turntables on which they are mounted, in rotating, do not describe a perfectly flat path. Records often become warped and rotating turntables often have undesirable wobbles. In the face of these problems, in order to prevent audio distortions, it is desirable that the stylus of the phonograph maintain a fairly constant pressure on the record groove. In addition to eratic over reaction of the tone arm to instabilities in the record surface, low frequency vibrations may be set up in the tone arm due to system feedback which also causes distortion in the reproduced sound.
From the above it may readily be seen that the most advantageous tone arm would be one which addresses the groove in a radial manner without side pressure and having a stylus which applies approximately constant, control pressure to the groove and does not suffer from unwanted, undamped vibrations.
In the prior art many attempts have been made to solve some or all of the above audio reproduction problems utilizing a variety of mechanical schemes. For example, many types of stylus pressure controlling devices have been used and grease-filled dashpots have been used to smooth out tone arm vibrations. A variety of spring loading and other mechanical schemes have been used to overcome skating and drag problems. Most of these schemes have met with only partial success.
Noting the limitations of mechanical control, the more recent prior art contains several attempts to control tone arm and stylus forces during the playing of a record electrically. One such apparatus is shown in a patent to Rabinow, U.S. Pat. No. 3,830,505 issued on Aug. 20, 1974. That patent illustrates and describes a system which uses arm positioning sensor coils in conjunction with an electric motor to control the stylus force. Another such system is shown in U.S. Pat. No. 3,623,734 to Sakamoto et al., issued Nov. 30, 1971. A magnetic damping system is shown in a patent to Haddy, U.S. Pat. No. 3,366,382 issued Jan. 30, 1968. Most of these systems require the monitoring and processing of feedback signals to achieve any control.
As cited in applicant's above-referenced, co-pending application, several attempts have also been made to produce linear tracking radial tone arms. The means for controlling these prior art radial tone arms have been cumbersome and ineffective. It thus appears, that it would be very desirable to combine the advantages of a very low friction radial tone arm with a simple, but accurate and repeatable tone arm control system.